The present invention relates to electrophotographic printing. More specifically, it relates to release agent compositions useful, for example, in conjunction with the hot fuser roll in an electrophotographic printing process.
In the process of electrophotography, the light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with a subsequent rendering of that latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of paper, with subsequent affixing of the image thereto.
In order to fix or fuse electroscopic toner material onto a support member permnanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner become tacky. This action causes the toner to flow to some extent into the fibers or pores of the support member (e.g., paper). Thereafter, as the toner material cools, solidification occurs causing the toner material to become bonded firmly to the support member. In electrophotography, the use of thermal energy for fixing toner images onto a support member is old and wellknown methodology.
One approach to thermal fusing of electroscopic toner images has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of this type of fusing system, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll, thereby heating the toner image within the nip. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the xe2x80x9chot offsetxe2x80x9d temperature of the toner at which temperature the toner particles in the image areas of the toner liquefy and cause a splitting action in the molten toner resulting in xe2x80x9chot offsetxe2x80x9d. Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface, such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e., xe2x80x9ccoldxe2x80x9d offsetting). This is generally caused by imperfections in the properties of the surface of the roll, or by the toner particles not adhering to the copy sheet as a result of insufficient adhesion forces. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the back-up roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or back-up roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus. The presence of such wayward toner particles can result in poor copy quality.
Most fusers of the type described above employ some method of applying a release fluid to the hot roll. Because of their inherent temperature resistance and release properties, silicone oils are typically used to prevent toner from adhering to the surface of the fuser roll and thereby degrading image quality and contaminating the fuser surface. The silicone oil also extends the life of the fuser rollers by providing some measure of lubrication to reduce the wear caused by the cumulative action of tens of thousands of pages passing through the pressure nip of the fuser. In order to ensure the positive effects of the release fluid, a minimum amount of oil (typically 10-500 xcexcg per page for most fuser systems) is required.
Since the oil used as a release agent is partially carried away by the paper passing through the fuser system, it is necessary to ensure that the amount of oil dispensed is not so much that objectionable print quality defects are seen. In extreme cases, the surface of the imaged page can become visibly wet or glossy with oil. In cases of duplex printing (i.e., printing on both sides of the page), a more subtle effect is seen. In such instances, oil is carried back though the printing process by duplexed pages and the oil on those pages is deposited on various machine surfaces, including the photoconductor. It has been found that minute amounts of oil, invisible to the eye, can be enough to drastically affect the transfer of toner from the developer roll to the photoconductor. Since the development process depends upon a scrubbing action between the toned developer and the imaged photoconductor to aid in the transfer of toner from the developer to the photoconductor, and since the scrubbing action is induced by a mismatch in surface speed between the developer and the photoconductor, the addition of silicone oil at the interface of the two surfaces reduces the frictional scrubbing force to a level where transfer of toner can be severely impaired. Such print quality defects are very apparent in fine resolution printing. In extreme cases, the lack of toner transfer is seen even in 12 point text as light print. Excessive levels of oil can cause severe print defects if the distribution of oil across the page is not uniform. Typical print quality defects are white streaks in gray scale, with the streaks parallel to the process direction in areas of high oil concentration.
A typical lubricant metering scheme employed in low-cost desk top printers involves saturating a felt pad constructed of temperature resistant material (such as DuPont""s NOMEXO fiber) with silicone oil of a viscosity such that, when combined with the fiber size and density of the felt, the rate of flow out of the felt can be controlled within reasonable limits. Typical construction of a wiper pad includes application of an amount of silicone oil(e.g., 7-8 grams) of viscosity about 30,000 cst (at room temperature) to a precut piece of felt (e.g., fiber diameter 9 microns, felt density=55 oz./yd2.), and baking the felt/oil combination at a high temperature to allow the oil to soak into the felt. Before high resolution printing (1200 DPI) and duplex printing, such a scheme was an excellent metering system, controlling oil flow within a range of 50-500 micrograms per page, with reasonable flow uniformity and no image defects. When this metering scheme is used with 1200 DPI and duplex printing, however, the previously acceptable non-uniform distribution of oil produces oil concentrations in some areas that are high enough to result in the above-described print defects.
Because of white streaks in gray scale, 100 xcexcg per page is generally the upper limit of a wiper pad with oil. To apply oil more uniformly, a felt roll may be used. The release agent delivery roll can either roll freely or be driven against the hot roll. The release agent is delivered to the surface continually via a reservoir at the center and the capillary action of the outer material. The roll shown in FIG. 1, is comprised of a high temperature paper or non-woven material (i.e., web) (1) which is wrapped around a metal core (2) and saturated with release agent. The reservoir material is then wrapped with a top layer of felt or metering membrane (3) to control the release agent flow from the system. In the preferred embodiment, the metal core shaft is made of aluminum and is 6 mm in diameter; the non-woven material is an aramid/polyester blend and is about 6-8 mm thick; and the metering layer is felt that is about 2-3 mm thick.
Another requirement of the oil application system is that the amount of oil dispensed must be consistent during the life of the applicator (typically for a wiper pad, about 14,000 pages and for a roll, about 50.000 pages). As previously mentioned, failure to maintain adequate oil flow causes toner to adhere to the fuser and reduces the life of the fuser. Also, low flow allows toner to be collected on the felt applicator; when enough toner accumulates, a mass of toner breaks free and adheres to the page, causing another print quality defect called xe2x80x9cwiper dump.xe2x80x9d The felt applicator is a gravity feed system. This means that oil flows out of the felt at a constant rate for a given temperature. Silicone oil continuously flows out of the wiper even if the printer is not printing and is at standby. Thus, if the printer is at standby for a sufficient amount of time, the first page printed will receive an abnormally large amount of silicone oil and show duplex streaks.
In summary, for optimum performance, the release agent oil application system must meet the following requirements:
Sufficient and consistent oil flow over the life of the system to prevent adherence of toner to the fuser roll. This extends fuser life and prevents wiper dumps. Minimum flow rate of 10 micrograms per page is required.
A maximum flow rate of 500 micrograms per page and uniform flow to prevent image defects when printing high resolution images at 1200 DPI and in duplex mode is required.
The present invention, as described below, provides silicone interpolymers useful in release agent compositions in electrophotographic printing processes.
U.S. Pat. No. 4,185,140 describes polymeric release agents for use on hot fuser rolls in an electrophotographic duplication process. The polymer materials utilized must include functional groups such as carboxy, hydroxy, isocyanate, thioether or mercapto groups. These materials are said to form a thermally stable release layer on the fuser roll which has excellent toner release properties. It is taught that the polymer material may be solid at room temperature, so long as it is a liquid at the temperature of the fuser. The materials disclosed as release agents are not silicone oils or waxes.
U.S. Pat. No. 5,959,056 describes release agent compositions useful in an electrophotographic process. These materials, when used in a wiper pad dispensing system cause streaks on transparencies, which shows up as black lines down the page. The copolymers described also are also useful, in the form of a paste or caulk, for sealing leaks in toner cartridges. Further, U.S. Pat. Nos. 5.952,442 and 5,880,244 describe release agent compositions and copolymers useful for sealing leaks in toner cartridges.
U.S. Pat. No. 5,929,186 teaches release agent compositions and copolymers useful for sealing leaks in toner cartridges, as well as a method for preparing such copolymers.